1. Field of the Invention
The present invention relates to an image forming apparatus and a control method thereof and a program.
2. Related Background Art
Conventionally, in the image forming apparatus such as a copying machine, a printer, and a facsimile which are an electrophotographic type using toner during image forming, a fan is attached to the image forming apparatus in order to exhaust heat and air to the outside of the image forming apparatus. The number of rotations (or rotating speed) of the fan is controlled in order to suppress problems, such as noise and power consumption increase, caused by the attachment of the fan.
For example, Japanese Patent Application Laid-Open No. 2003-270890 discloses a method of changing the number of rotations of the fan depending on color image printing and monochrome image printing.
Japanese Patent Application Laid-Open No. 2004-117732 discloses a method, in which temperature detection means (thermistors) are provided in the image forming apparatus or peripheries of a fixing device and a transfer portion and the number of rotations of the fan is changed based on detection temperatures detected by the thermistors.
In the case of the use of the method disclosed in Japanese Patent Application Laid-Open No. 2004-117732, the fan is rotated at a half of normal speed when a print job is started, and the fan is switched to be rotated at the normal speed when the detection temperatures of the temperature detection means reach a predetermined threshold value, which suppresses the excessive temperature increase caused by the long time printing in the image forming apparatus. Accordingly, the method proposed in Japanese Patent Application Laid-Open No. 2004-117732 is effective from viewpoints of silence and electric power saving.
However, when the number of rotations of the fan is controlled based on the detection temperature detected by the temperature detection means as described above, because the temperature is detected near a position where the temperature detection means is installed, it is difficult to grasp the temperature of a component for which cooling is actually required in the image forming apparatus.
A main reason why the temperature of the component is increased in the image forming apparatus is that the heat generated from the fixing device which is of a heat source in the image forming apparatus diffuses with time. In the component arranged in a conveying path, the temperature is increased by direct heat transfer from the fixing device, and the temperature is also increased by indirect heat transfer from a recording sheet during conveyance through a conveying path. The recording sheet is heated by passage through the fixing device. Further, in the image forming apparatus with a recording sheet reverse mechanism in which the images can sequentially be formed in both the surface and the back side of the recording sheet, when compared to printing in which only single-side printing is continuously performed, the temperature tend to be increased in the case in which the printing including double-side printing is continuously performed.
In the image forming apparatus, a sheet discharging roller holder of a sheet discharging portion which discharges the recording sheet to the outside of the apparatus can be cited as the component in which the temperature is particularly easily increased during the printing. As shown in FIG. 7, a sheet discharging roller holder 703 nips the recording sheet with a sheet discharging roller 701 driven by a motor. The sheet discharging roller holder 703 supports a sheet discharging roller 702 which is rotated in conjunction with movement of the recording sheet during the sheet discharging. The sheet discharging roller holder 703 has a structure in which the sheet discharging roller holder 703 is vertically movable by a spring in order to smoothly performing the sheet discharging.
A material such as polycarbonate having relatively low heat-resistant temperatures ranging from 90° C. to 115° C. is used for the sheet discharging roller holder in order to reduce cost. On the other hand, because the temperature exceeds 200° C. during the printing in the fixing device which is of the heat source in the image forming apparatus, as shown in FIG. 8, the sheet discharging roller holder exceeds the heat-resistant temperature when the double-side continuous printing exceeds 120 minutes at a room temperature of 23° C. Therefore, when the print job including the double-side printing is performed for a long time, the sheet discharging roller holder is deformed and the recording sheet is not smoothly discharged, which results in the problems such as paper jam in the worst case.
In order to prevent the troubles, there is the method in which the rotating speed of the fan is increased compared with the normal speed in the case of the long time printing. However, at the room temperature of 23° C., the temperature increase in the single-side continuous printing is moderate when compared with the double-side continuous printing (see FIG. 8), so that the cooling fan is excessively driven when the single-side continuous printing is performed for a long time.